A manhole is an opening, often with a cover, through which a person can enter a sewer, conduit, tank, underground tunnel, etc. for a repair work or an inspection.
As shown in FIGS. 1 and 2, a conventional manhole cover comprises an annular base 1 and a cover 2 so dimensioned to fit into the annular base 1, which is provided with an annular groove 3 serving to receive therein a flange 4 of the cover 2. With a view to facilitating the removal of the cover 2 from the base 1, the furrow width of the annular groove 3 is greater than the thickness of the flange 4. In other words, the manhole cover 2 is not received in the annular base 1 in a watertight manner. As a result, the annular groove 3 can be easily inundated with the water, which can not be easily drained and which causes the corrosion of the manhole and even the. flooding of the facilities located under the manhole.